1. Field of the Invention
The present invention relates to a glass substrate for an information recording medium such as a magnetic disk or the like, and more particularly to a highly acid-resistant glass substrate having an excellent level of surface smoothness.
2. Description of the Related Art
Generally, aluminum substrates are used as substrates for use in information recording mediums. However, since it is difficult for aluminum substrates to increase their surface smoothness, the aluminum substrates are not suitable for use in disks for high-density recording. Instead, glass substrates which have a high level of surface hardness and which can be polished to a high level of surface smoothness are used in disks for high-density recording.
Glass substrates for use in information recording mediums are fabricated by forming a molten material according to a float process, a rod process, and a press process, cutting the formed material to a desired substrate shape, if necessary, and thereafter polishing the substrate to adjust its thickness and surface smoothness to a predetermined range. If desired, the polished substrate is strengthened by an ion exchange or the like. Consequently, fine particles of foreign matter, primarily a polishing compound, remain attached to the surface of the substrate. While those fine particles of foreign matter cannot completely be removed from the substrate by an ordinary cleaning process, they have heretofore posed no significant problems. However, the present trend toward high-density information recording mediums inevitably demands a smaller gap between the information recording medium and the head for reading information from and writing information in the information recording medium. The fine particles of foreign matter need to be removed from the substrate as completely as possible because they would tend to hit the head and cause an error or crash because of the small gap between the information recording medium and the head. Though complete removal of fine particles of foreign matter from the substrate cannot easily be performed, they can efficiently be removed when they are dissolved by an acid such as sulfuric acid, hydrofluoric acid, etc., or the surface of the substrate is etched.
Japanese patent publication No. 46-4271 discloses a chemically strengthened glass composed of 1 to 5 weight % of MgO, 0 to 5 weight % of K2O, 5 to 25 weight % of Na2O, 5 to 25 weight % of Al2O3+ZrO2, and SiO2, the sum of which is 80% or more of the entire composition. However, the disclosed glass is poor in acid resistance and hence cannot be treated by an acid for the removal of foreign matter therefrom.
Japanese laid-open patent publication No. 5-32431 discloses another chemically strengthened glass of improved acid resistance which is composed of 62 to 75 weight % of SiO2, 5 to 15 weight % of Al2O3, 4 to 10 weight % of Li2O, 4 to 12 weight % of Na2O, and 5.5 to 15 weight % of ZrO2, with the weight ratio of Na2O/ZrO2 being in the range from 0.5 to 2.0 and the weight ratio of Al2O3/ZrO2 being in the range from 0.4 to 2.5. The publication, however, fails to show a causal dependence of the acid resistance on surface irregularities. Furthermore, since the glass revealed in the publication contains a large amount of ZrO2, ZrO2 tends to be separated out as fine crystals in the glass, which project on the surface after it is polished and are liable to cause an error and crash.
Japanese laid-open patent publication No. 62-187140 reveals still another chemically strengthened glass composed of 64 to 70 weight % of SiO2, 14 to 20 weight % of Al2O3, 4 to 6 weight % of Li2O, 7 to 10 weight % of Na2O, 0 to 4 weight % MgO, and 0 to 1.5 weight % of ZrO2. However, the publication also fails to show a causal dependence of the acid resistance on surface irregularities.
Japanese laid-open patent publication No. 9-22525 discloses a process of cleaning a glass substrate pulled up out of a strengthening liquid with a solution containing an acid, in the fabrication of a glass substrate for use in a magnetic disk. The disclosed process is aimed at only the removal of a chemically strengthening salt. The publication is silent as to the acid resistance which the glass substrate is required to have, and does not suggest or describe the level of surface smoothness in question.
It is therefore an object of the present invention to provide a glass substrate for an information recording medium which can easily be treated by an acid to achieve high cleanliness and high smoothness.
To achieve the above object, there is provided in accordance with the present invention a glass substrate for an information recording medium having at least one surface as a recording surface, the glass substrate having an acid resistance represented by an etching rate of at most 45 nm/min. upon contact with a hydrofluoric acid having a temperature of 50xc2x0 C. and a concentration of 0.1 weight %, at least the recording surface having an average surface roughness Ra smaller than 0.3 nm.
When glass is etched by an acid, the components of the glass are not uniformly dissolved, but those components which are weaker on the acid are dissolved at first, and then those stronger on the acid are dissolved gradually. The components which are weaker on the acid include alkali, alkaline earth group, alumina, etc., and those stronger on the acid include silica, zirconia, titania, etc. The surface of the glass treated with the acid, as microscopically observed, has a porous layer of network structure mainly composed of silica, zirconia, titania, etc. When the substrate thus produced is cleaned by an alkali for the purposes of removing the porous layer and fats and oils, since the porous layer is thick and unevenly dissolved by the alkali if the substrate is low in acid resistance, pores appear on the surface, producing surface irregularities which make it impossible to achieve a level of surface smoothness that is required by the information recording medium. It has been found according to the present invention that in order to keep the surface smoothness (average surface roughness) Ra of the substrate in the range of Ra less than 0.3 nm when cleaned by the alkali after having been treated with the acid, the etching rate of the substrate glass with an aqueous solution of 0.1 weight % of hydrofluoric acid at a temperature of 50xc2x0 C., as an indication of acid resistance, needs to be 45 nm/min. or less.
An acid liquid used in the present invention serves to dissolve a polishing compound on the glass substrate or etch the surface of the glass substrate to remove a polishing compound therefrom. The acid liquid may include an inorganic acid such as hydrofluoric acid, a mixture of hydrofluoric acid and ammonium fluoride, hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, etc., or an organic acid such as sulfamic acid, formic acid, oxalic acid, citric acid, malic acid, hydroxyacetic acid, gluconic acid, etc.
A polishing compound mainly composed of cerium oxide, which is used most generally, can be removed most efficiently when it is dissolved in an aqueous solution of sulfuric acid. If the concentration of the sulfuric acid used to remove the polishing compound were less than 0.01 weight %, then the removing ability would be insufficient, and if the concentration of the sulfuric acid exceeded 5 weight %, then fine defects of the glass substrate would appear on the surface. Therefore, the concentration of the sulfuric acid should preferably in the range from 0.01 to 5 weight %. In order to keep a desired level of surface smoothness under this condition, the etching rate of the substrate glass with an aqueous solution of 0.1 weight % of hydrofluoric acid at a temperature of 50xc2x0 C. should preferably be 45 nm/min. or less.
Even if a polishing compound other than cerium oxide is used, the attached polishing compound can be removed irrespective of its type by etching the substrate surface with an acid having a high etching effect, such as hydrofluoric acid. If the concentration of the hydrofluoric acid used to remove the polishing compound were less than 0.002 weight %, then the removing ability would be insufficient, and if the concentration of the hydrofluoric acid exceeded 1 weight %, then fine defects of the glass substrate would appear on the surface. Therefore, the concentration of the hydrofluoric acid should preferably in the range from 0.002 to 1 weight %. In order to require the glass to have a high level of acid resistance and keep a desired level of surface smoothness, the etching rate of the substrate glass with an aqueous solution of 0.1 weight % of hydrofluoric acid at a temperature of 50xc2x0 C. should preferably be 30 nm/min. or less.
The composition of the glass substrate which meets the requirement for the etching rate of 45 nm/min. or less with the aqueous solution of 0.1 weight % of hydrofluoric acid is preferably as follows from the standpoint of glass solubility and substrate weather resistance: In terms of molar fractions, the difference between SiO2 and Al2O3 (SiO2xe2x88x92Al2O3): 56.5%, SiO2: 63-70%, Al2O3: 4-11%, Li2O: 5-11%, Na2O: 6-14%, K2O: 0-2%, TiO2: 0-5%, ZrO2: 0-2.5%, RO: 2-15% (RO=MgO+CaO+SrO+BaO, MgO: 0-6%, CaO: 1-9%, SrO: 0-3%, BaO: 0-2%), and other components: 3% or less. The glass composition will hereinafter be expressed in terms of molar fractions unless otherwise specified.
The composition of the glass substrate which meets the requirement for the etching rate of 30 nm/min. or less with the aqueous solution of 0.1 weight % of hydrofluoric acid is preferably substantially the same as the above composition except that the difference SiO2xe2x88x92Al2O3 is 58.5%, from the standpoint of glass solubility and substrate weather resistance.
From the standpoint of solubility, a glass composition similar to the above composition except that TiO2: 0-3% and ZrO2: 0-2% is more advantageous as the devitrification temperature is lower.
The reasons for the limitations of the glass composition are as follows:
The etching rate based on the aqueous solution of 0.1 weight % of hydrofluoric acid depends strongly on the difference SiO2xe2x88x92Al2O3 as expressed by a molar fraction. If the difference SiO2xe2x88x92Al2O3 were less than 56.5%, then the alkaline component would be required to be greatly reduced in order to make the etching rate equal to or less than 45 nm/min. through adjustments of other components. The resultant composition would have a high dissolving temperature, and would not easily be chemically strengthened by an ion exchange.
The same problem would arise if the etching rate were 30 nm/min. or less with the difference SiO2xe2x88x92Al2O3 being 58.5% or less. Therefore, in order to make the etching rate equal to or less than 45 nm/min. or 30 nm/min., the difference SiO2xe2x88x92Al2O3 should preferably be 56.5% or 58.5% or higher.
SiO2 is a major component of the glass. If the proportion of SiO2 were less than 63%, then the chemical durability of the glass would be lowered. If the proportion of SiO2 were in excess of 70%, then the viscosity would increase to the extent that it would be difficult to melt the glass. Therefore, the proportion of SiO2 should preferably range from 63% to 70%.
Al2O3 is a component for increasing the depth of a compressive stress layer due to an ion exchange and also increasing the water resistance of the glass. If the proportion of Al2O3 were less than 4%, the above effects would not sufficiently be developed. If the proportion of Al2O3 were greater than 11%, the viscosity would increase and the liquid phase temperature would increase more than the viscosity, resulting in a reduction in solubility. Therefore, the proportion of Al2O3, should preferably range from 4% to 11%, and more preferably range from 6% to 11%.
Li2O is a component to be exchanged in an ion exchange and to increase the solubility. If the ratio of Li2O were less than 5%, the compressive stress after the ion exchange would become insufficient and the viscosity would increase to the extent that it would be difficult to melt the glass. If the ratio of Li2O exceeded 11%, then the weather resistance and acid resistance of the substrate would be lowered. Therefore, the proportion of Li2O should preferably range from 5% to 11%.
Na2O is a component to be exchanged in an ion exchange and to increase the solubility. If the ratio of Na2O were less than 6%, the compressive stress after the ion exchange would become insufficient and the viscosity would increase to the extent that it would be difficult to melt the glass. If the ratio of Na2O exceeded 14%, then the weather resistance and acid resistance of the substrate would be lowered. Therefore, the proportion of Na2O should preferably be equal to or less than 14%.
K2O is a component to increase the solubility. If the proportion of K2O exceeded 2%, then the weather resistance would be lowered, and the surface compressive stress after the ion exchange would be reduced. Therefore, the proportion of K2O should preferably be equal to or less than 2%.
TiO2 is a component to increase the weather resistance of the glass. If the proportion of TiO2 exceeded 5%, then the liquid phase temperature of the glass would rise and the devitrification resistance would be lowered. Therefore, the proportion of TiO2 should preferably be equal to or less than 5%, and more preferably be equal to or less than 3%.
ZrO2 is a component to increase the weather resistance of the glass. If the proportion of ZrO2 were in excess of 2.5%, the possibility for ZrO2 to be separated out as fine crystals when melted would be increased. Therefore, the proportion of ZrO2 should preferably be equal to or less than 2.5%, and more preferably be equal to or less than 2%.
MgO is a component to increase the solubility of the glass. If the proportion of MgO were in excess of 6%, then the liquid phase temperature of the glass would rise and the devitrification resistance would be lowered. Therefore, the proportion of MgO should preferably be equal to or less than 6%, and more preferably be equal to or less than 4.5%.
CaO is a component to increase the solubility and the thermal expansion coefficient of the glass. If the proportion of CaO were less than 2%, then the effect to increase the solubility and the thermal expansion coefficient of the glass would be small. If CaO existed together with either of SrO or BaO, or both of SrO and BaO, then the effect would be sufficiently produced in the proportion equal to or more than 1%. If the proportion of CaO were in excess of 7.5%, then the liquid phase temperature of the glass would tend to rise. If the proportion of CaO were in excess of 9%, then such tendency would be remarkable. Therefore, the proportion of CaO should preferably be in the range from 1 to 9%, more preferably be in the range from 2 to 7.5%.
SrO is a component to increase the solubility and the thermal expansion coefficient of the glass. The effect of SrO to increase the thermal expansion coefficient is higher than that of CaO. However, if the glass contained a large amount of SrO, then the specific gravity of the glass would increase. The proportion of SrO should preferably be equal to or less than 3%.
BaO is a component to increase the solubility and the thermal expansion coefficient of the glass. The effect of BaO to increase the thermal expansion coefficient is higher than that of CaO and SrO. However, if the glass contained a large amount of BaO, then the specific gravity of the glass would increase. The proportion of BaO should preferably be equal to or less than 2%.
If the proportion of the sum of MgO+CaO+SrO+BaO (RO) were smaller than 2%, then the solubility would be insufficient. If the proportion of the sum would exceed 15%, then the liquid phase temperature of the glass would rise and the devitrification resistance would be lowered. Therefore, the proportion of the sum (RO) should preferably range from 2 to 15%, and more preferably range from 2 to 12%.
In addition to the above components, other components including As2O3, Sb2O3, SO3, SnO2, Fe2O3, CoO, Cl, F, etc., for example, may be added in a total proportion up to 3% for the purpose of cleaning the glass when melted.
Since the glass substrate contains lithium, an ion exchange may be performed in a molten salt including ions of potassium, sodium, or both for imparting compressive stresses to the glass surface to increase the fracture strength.
An information recording medium which is fabricated using the above glass substrate has a high level of surface smoothness and is free of surface projections due to foreign matter. Therefore, the height of a magnetic head which is lifted from the surface of the information recording medium can be reduced for recording information at a high density on the information recording medium.
The treatment with the acid described above can be carried out anytime after a process of polishing the glass substrate to a mirror finish and immediately before the growth of a recording layer on the glass substrate.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.